Non-Inverting Operational Amplifier | Non-Inverting Op Amp

In this lecture, we are going to discuss the Non-Inverting Operational Amplifier. We will discuss the operation of a non-inverting Op Amp, the derivation of the closed-loop voltage gain of the non-inverting operational amplifier, and input and output resistance.

There are three basic op-amp configurations:

1. Inverting operational amplifier
2. Non-inverting operational amplifier
3. Buffer amplifier or Voltage follower

Note that the Non-inverting operational amplifier configuration discussed in this section, we assume that the used operational amplifier in this section is ideal.

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Non-Inverting Operational Amplifier

The non-inverting operational amplifier using Op-amp is shown in the below figure. Here the signal that is to be amplified is applied to the non-inverting (+) terminal of the Op-amp.

As shown in the figure, the input and output voltages are in phase with each other.

The negative feedback is incorporated in this circuit CIA the feedback resistor RF which is connected between the output and inverting terminal of Op-amp.

Assume the op-amp is ideal.

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Closed Loop voltage gain of Non-inverting Op-amp

As we are using the ideal op-amp, R_i = ∞. Therefore the current entering into both the input terminals of op-amp will have a zero value. (I₁ = I₂ = 0)

Therefore voltage across R₁ is given by,

\mathbf{{V_2 = \frac{R_1}{(R_F + R_1)}V_0}}

As per the virtual short concept discussed earlier,

V₂ = V₁ = V_s

\therefore \mathbf{{V_s = \frac{R_1}{(R_F + R_1)}V_0}}

Therefore the closed loop voltage gain A_VF is given as,

\mathbf{A_{VF} = \frac{V_0}{V_s} = \frac{R_1 + R_F}{R_1}}

\therefore \boxed{\mathbf{A_{VF} = 1 + \frac{R_F}{R_1}}}

Conclusions from the Expression of A_VF:

1. The positive sign of the equation indicates that the input and output are in phase with each other.
2. The closed-loop control gain is always greater than unity (1).
3. A_VF is adjustable and its value can be adjusted by varying the values of R_F and R₁. Generally, a variable resistor is used in place of R_F to adjust the closed loop gain to its desired value.
4. A_VF is independent of the open loop gain of the op-amp. It depends only on the values of R_F and R₁.

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Ideal Closed Loop Characteristics of Non-inverting Op Amp

We have already obtained the expression for closed loop gain of the non-inverting operational amplifier. Let us now obtain the values of closed-loop input and output resistances.

Closed loop input resistance R_iF

• The closed loop input resistance of the non-inerting op-amp can be obtained from the ideal equivalent circuit shown in the figure below.

• The closed loop input resistance R_iF is the resistance seen by the source between the non-inverting (+) terminal and ground.

• Due to the infinite input resistance (R_i = ∞) of the ideal op-amp the non-inverint (+) terminal appears as an open circuit.

∴ R_iF = ∞

• Note that the input resistance is ∞, and not R1 as for the inverting amplifier. This is the advantage of using the non-inverting amplifier.

Closed loop input resistance R_oF

• As the ideal op-amp is being used, its output resistance R0 = 0. Hence the output resistance of the closed loop configuration is 0.

∴ R_0F = 0

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FAQs on Non-inverting Op Amp

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